In oil exploration and geophysical survey, it is useful to provide measuring systems which can detect locations of boundaries between different formations. Examples of applications of such systems include reservoir characterization, geo-steering in horizontal drilling, salt dome mapping for engineering of gas storage caverns and delineation of hydrocarbon traps, waste disposal control, and rock fracture detection in environmental logging. There are several advantages of using transient electromagnetic (radar) signals in such measurements. First, the arrival time of reflected signals is a direct measure of the distance of boundary locations from the borehole, if the wave speed of the background medium is known. Second, the pulse can have a narrow temporal width which gives high spatial resolution.
As well as the distance of a structure from the borehole, it is also desirable to know the azimuthal direction of the structure relative to the borehole. A borehole radar logging tool is proposed in U.S. Pat. No. 4,814,768 in which this direction is measured. This tool has a directional transmitting antenna and a corresponding directional receiving antenna. Schematic views of the directional antennas are shown in FIGS. 1(a) and l(b). The antenna comprises an elongated conductive radiator element 10 mounted inside the angle of a pair of reflective plates 12, 14 which form a corner reflector. The tool is designed to operate in the 30MHz to 300MHz frequency range with a pulse repetition rate of more than 10 kHz. The electromagnetically active portion 16 of the antenna is filled with a dielectric material to shorten the wavelength so that the antenna will be directional at the desired frequencies. This material is typically a mixture of barium titanate and air. The transmitter and receiver are aligned in the tool such that reflected signals from a given direction only are received. The tool also includes some means to rotate the antennas so as to point in a desired direction and a direction indicator such as a flux gate compass.
It is an object of the present invention to provide a borehole radar tool which does not require rotation of the antennas to give azimuthal sensitivity for all directions.
It is a further object of the invention to provide a directional antenna which does not rely solely on a corner reflect for its directionality.
The present invention achieves this object by providing a plurality of antenna elements at different positions on the tool and measuring the difference in signals measured between the elements to obtain the directional information.